The present invention relates to a high-molecular weight, linear copolyesters and in particular, a poly(butylene terephthalate) (PBT) copolymer and a process for preparing it by the condensation of 40-85 molar percent of terephthalic acid or an ester thereof which component can be substituted to up to 50% by other dicarboxylic acids, and 60-15 molar percent of adipic acid with alkanediols having 2-6 carbon atoms in the carbon chain, in 2 stages at an elevated temperature in the presence of conventional catalysts.
Such copolyesters, as well as their manufacture, are known in principle and are disclosed in U.S. Pat. No. 4,328,059 which is incorporated by reference.
At high temperatures required for the polycondensation, especially in the second condensation stage (polycondensation stage), the presence of adipic acid results in final products which have a pinkish discoloration. This is unacceptable to the processor of these products Therefore, the addition of esters and salts of phosphoric acids in conjunction with talc has been proposed (DOS No. 2,703,417). At a low adipic acid content of the copolyesters (&lt;15 molar percent based on the total acid component), these measures partially improve the color of the finished products. At higher adipic acid contents, the undesirable discolorations cannot be avoided to an adequate extent.
Adipic-acid-containing copolyesters exhibit an additional grave deficiency. In the melt, they do not show adequate stability against degradation by thermal oxidation. Yet, such stability is absolutely required, for example, in hot-melt [fusion] adhesives, since the latter are kept in open melt tanks during their processing for a prolonged period of time.